1. Field of the Invention
The present invention relates to an electric power steering device that generates a steering assist torque with a motor.
2. Description of the Related Art
Up to now, there has been proposed a control method in which a torque sensor that detects a steering torque of a steering system and a vehicle speed sensor that detects a vehicle speed are disposed, and the assist torque of a motor is controlled based on the outputs of those sensors, and the inertia and viscosity of a steering system are compensated based on the angular velocity and the angular acceleration of the motor in addition to the above outputs in order to improve the steering feeling in an electric power steering device.
In the conventional electric power steering device, as described above, a target current is determined according to a target motor torque which is determined based on the steering torque, and the angular velocity and angular acceleration of the motor, and a current feedback control is conducted so that a detection current that is detected by a shunt resistor that is inserted in a current path of the motor coincides with the target current.
In the conventional electric power steering device, since the current feedback control is implemented as described above, the target current and the detection current coincide with each other when the device is normal. Therefore, it is determined that the device is abnormal when a difference between the target current and the detection current becomes equal to or higher than a given value.
In the conventional electric power steering device described in JP 06-29031 B, there is provided a shunt resistor for detecting the current. A current flows in the shunt resistor to generate a heat in the shunt resistor.
In recent years, the types of vehicles in which the electric power steering device is installed are expanded to a standard sized car that is large in weight, and a current of the motor is increasingly made larger, and a problem on the heat generation due to the shunt resistor becomes remarkable. Also, a voltage drop due to the shunt resistor can be put into a power loss due to the shunt resistor, and with a larger current in recent years, the power loss due to the shunt resistor also increases, resulting in such a problem that the higher efficiency of the device is impeded.
Also, in order to radiate the heat caused by the shunt resistor, it is necessary to provide a controller with a large-sized heat sink. Also, in order to detect the current, there is required a peripheral circuit for loading a voltage between both ends of the shunt resistor into a microcomputer as well as the shunt resistor. For that reason, not only the controller gets complicated, but also a space for mounting those parts on a circuit board is required, resulting in such a problem that the controller becomes large, and the costs are increased.